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Chapter 7

Modern Chemistry of the Ancient Chemical Processing of Organic Dyes and Pigments Downloaded by CORNELL UNIV on August 22, 2016 | http://pubs.acs.org Publication Date (Web): November 20, 2015 | doi: 10.1021/bk-2015-1211.ch007

Zvi C. Koren* The Edelstein Center for the Analysis of Ancient Artifacts, Department of Chemical Engineering, Shenkar College of Engineering, Design and Art, Ramat-Gan 52526, Israel *E-mail: [email protected]

The ancient dyer was an advanced empirical chemist. While inorganic pigments produced magnificent colors, the most elaborate chemical processing of colorants in antiquity – from the source to the final product – involved organic dyes from flora and fauna sources. Towards this end, the dyer applied his – or her – practical chemical knowledge to botany, entomology, and malacology. By controlling the temperature and the alkaline or acidic pH of the dye bath, the dyers were able to create colorful textile dyeings with some surviving even after six millennia. In order to produce stable products, the ancient dyer mastered the methods that are based on advanced chemical topics, such as, ionic, covalent, and intermolecular bonding, coordinate complexation, enzymatic hydrolysis, photochemical chromogenic precursor oxidation, anaerobic bacterial fermentative reduction, and redox reactions. This paper discusses various chemical principles that were applied by the ancient master of colorful chemistry.

© 2015 American Chemical Society Rasmussen; Chemical Technology in Antiquity ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Downloaded by CORNELL UNIV on August 22, 2016 | http://pubs.acs.org Publication Date (Web): November 20, 2015 | doi: 10.1021/bk-2015-1211.ch007

Introduction: Historical Overview Two decades have elapsed since the first paper on the subject of ancient Near East dyestuff sources was published in an ACS Symposium Series by this author (1). In that work, emphasis was given to the yellow, red, and blue colorants that were obtained from flora dyestuff sources and the molecular structures of their main dyes were discussed. In the last two decades, more analyses of archaeological dyes have been performed with greater emphasis on the more important fauna origins of the red dyestuffs from scale insects and the purple and violet pigments from sea snails. This current paper, besides being a continuation and update of our knowledge in this field, focuses on the ancient dyer’s advanced level of control and the highly complex chemistry applied in the process. A number of classical English-language books have treated the history of organic dyes and pigments from botanical, entomological, and malacological sources used in ancient times. Some of the more prominent texts discussing these flora and fauna dyestuff sources include, chronologically, the works of Leggett (2), Forbes (3), Robinson (4), Brunello (5), and Sandberg (6, 7); and more recently by Cardon (8), Balfour-Paul (9), and Orna (10). The ancient and historical literature on dyestuffs includes the Natural History in Latin of the 1st century CE Roman historian, Pliny the Elder, and the 16th century Plictho of Gioanventura Rosetti in Italian (11). In fact, the full title of The Plictho, as translated into English, is: “Instructions in the Art of the Dyers Which Teaches the Dyeing of Woolen Cloths, Linens, Cottons, and Silk by the Great Art as Well as by the Common”. It is practically impossible to determine the geographical origin of textile dyeing. To the question of “where in the world was dyeing first performed?” one undoubtedly needs to answer that in all probability it was performed in various locations at about the same time by using the locally available dyestuff sources. Today, we have definitive archaeological and chemical evidence that this craftsmanship dates back at least six millennia, and these textile dyeings are the oldest yet found to date, and were excavated from the Cave of the Warrior in the Judean Desert, Israel (12). In these Chalcolithic linen textiles, the dark-brown (blackish) decorative yarns were dyed with an acidic organic macromolecule dyestuff source, but whose exact nature has yet to be determined, prior to weaving them into the textile. As for the famous purple molluskan colorant, the production of this water-insoluble compound as a paint pigment dates from about four millennia ago while its conversion to a soluble textile dye probably occurred about half a millennium later (13). Similarly, the use of the dark blue-violet indigo pigment (also known as indigotin) is from over four thousand years ago as analyzed by this author and observed from various surviving Pharaonic textile fragments in the collections of, for example, the British Museum in London and the Metropolitan Museum of Art in New York. The primary thrust of the current work is to portray a picture of marvel at the scientific abilities of the ancient dyer, undoubtedly achieved via empirical trial and error experimentation, and probably with an added dose of accidental successes. He – or she – utilized vast empirical know-how to produce colorful long-lasting dyeings that have withstood the ravages of time. When the dyer used the full 198 Rasmussen; Chemical Technology in Antiquity ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

spectrum of the natural dyestuff sources available, he also applied his practical knowledge of botany, entomology, and malacology. Further, he incorporated into his scientific craft the methodologies generally associated with the following topics in inorganic, organic, physical, and biochemistry:


• • • • • • •

ionic, covalent, and intermolecular chemical bonding coordinate complexation pH control of alkalinity/acidity enzymatic hydrolysis photochemical chromogenic precursor oxidation anaerobic bacterial fermentative reduction air-oxidation

A discussion of the colorants used in ancient – and modern – times requires a clear understanding of the difference between a “pigment” and a “dye”, which unfortunately have sometimes been erroneously used interchangeably. Figure 1 outlines the properties of these two colorants. Without any additional treatment, a natural pigment is a relatively water-insoluble colorant and is used to paint a surface. Examples are painting on a wall (as in a fresco), canvas (paintings), vessel, on a body, and even to paint on a textile (if that textile will not be washed such as a shroud on a coffin). In ancient times, most paint pigments were of an inorganic, mineral, nature. Conversely, a dye is a water-soluble organic colorant and that word should be specifically used when this colorant is utilized to perform a true textile dyeing. Historically, some dyes were chemically transformed into paint pigments by complexing with a metallic ion to form a “lake” used in various historic paintings or in mordant dyeing – indirectly fixing the dye into the textile (discussed below). Inversely, certain pigments (such as indigo from plants and related compounds from mollusks) were transformed into a dye by, for example, reducing the pigment to its water-soluble counterpart, and after the dyeing this was then followed by air-oxidation back to the original pigment.

Figure 1. The properties of water-insoluble pigments vs. soluble dyes. 199 Rasmussen; Chemical Technology in Antiquity ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Ancient Coordination Compounds Historical Mordanting with Alum


It was well known at least as far back as the Roman Period that a salt was needed in order to act as an intermediary and fix the dye to the textile fibers. This fixative is known as a “mordant” from the Latin mordere, which means “to bite” or “biting”, and it “bites” (i.e., attaches to) the dye as well as to the textile fiber. The Roman historian Pliny clearly mentions the use of the mordant alum in dyeing, and this mineral contains the trivalent aluminum ion and is generally accepted to be KAl(SO4)2·12H2O. The relevant passages in Pliny are from Book 35, Chapter 52, as follows in Latin and in English (14, 15): Nec minor est aut adeo dissimilis aluminis opera, quod intellegitur salsugo terrae. plura et eius genera. in Cypro candidum et nigrius, exigua coloris differentia, cum sit usus magna, quoniam inficiendis claro colore lanis candidum liquidumque utilissimum est contraque fuscis aut obscuris nigrum. Not less important or very different is the use made of alum (aluminis), by which is meant a salt exudation from the earth. There are several varieties of it. In Cyprus there is a white alum and another sort of a darker color, though the difference of color is only slight; nevertheless the use made of them is very different, as the white and liquid kind is most useful for dyeing woolens a bright color whereas the black kind is best for dark or somber hues. Complexing the Dye with Alum Alizarin and the related purpurin are the well-known di- and trihydroxyanthraquinone hydrolysis products extracted from various madder plants (Rubiaceae species), with the most famous being Dyer’s Madder (Rubia tinctorum), shown in Figure 2. This plant was the most widely used red dyestuff source of the ancient Near East and also available in various European regions. In wool, the Al3+ ion – a Lewis acid – bonds to an oxygen atom in the proteinic fiber and also to certain oxygen atoms in the dye molecule via ionic and covalent bonds, the latter including coordinate covalent bonding. Thus, in the schematic O(wool)–Al–O(dye) bridge, the Al intermediary serves as a bridging agent – a “matchmaker” – between the fibers and the dye. However, the exact molecular structure of this wool–alum–dye complex is still not known, and further research into this area would be important. Without wool, the structures of alizarin-mordant complexes have been studied for bivalent and trivalent metal mordants and two of the proposed general structures are shown in Figure 3. In order for the C=O → M–O coordination bond to form in the hydroxyanthraquinones, the carbonyl and hydroxyl groups in the ligand must be adjacent. In the structure proposed for a metal–alizarin complex in neutral media, Figure 3 (left), the hydrogen atom is replaced by the metal ion, which is followed by the ionization of the M–O bond, and the ionized hydroxyl 200 Rasmussen; Chemical Technology in Antiquity ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


group in position 1 is bonded through the intramolecular hydrogen bond (16). This complex can have more tautomeric forms that contain the six-membered chelate cycle, and other structures have been proposed for the metal–alizarin complex in acidic and alkaline media (16). The metal-alizarin coordination compound in Figure 3 (right) has often been given (1, 17), in which coordinate covalent bonds are formed; however, the M–O bond in similar structures has been found to be partly covalent (18), so that a partial ionic character can be present, and even a more fully ionized bond may exist, as mentioned above (16). Resonance structures are also possible for this 2:1 complex.

Figure 2. Left: roots of the Dyer’s Madder plant (Rubia tinctorum) with the interior showing the presence of the red dyestuff source. Right: molecular structures of alizarin (top) and purpurin (bottom); intramolecular H-bonds are also present between adjacent carbonyl and hydroxyl groups in both molecules.

Further studies have also included calcium ions into the complex, Figure 4, which is a logical extension since in antiquity naturally hard water rich in calcium was used. Further, the aluminum-calcium-alizarin complex is the dye known as Turkey Red for dyeing cotton (18). It was assumed that the groups around the central aluminum take on an octahedral structure (18). It is also noted that the two anionic oxygens are probably cis to each other in order to neutralize the Ca2+ counterion on their side. If this complex would be attached to the wool then probably the water molecule would be replaced by the oxygen from wool. 201 Rasmussen; Chemical Technology in Antiquity ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 3. Proposed alizarin complexes with an M2+ or M3+ metal ion mordant.

Figure 4. Alizarin–aluminum complex with calcium based on the structure proposed by Kiel & Heertjes (18). 202 Rasmussen; Chemical Technology in Antiquity ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Another elaborate structure for the alizarin–aluminum–calcium complex has been given and shown in Figure 5 (19).

Figure 5. A proposed alizarin–aluminum –calcium complex. (courtesy of Mattern, R.; Wikimedia Commons). Finally, Wunderlich and Bergerhoff (20) obtained single crystals of aluminum calcium alizarinate and purpurinate by two-phase crystallization. X-ray structure determinations showed tetranuclear complex molecules with four alizarins. The metal–oxygen interactions in this structure (Figure 6) are mostly coordinate covalent bonds with some ionic character. This structure is different from other proposed arrangements, and the authors state that this is the best structure. A similar complex is also given for carminic acid, which is also a hydroxyanthraquinone, and obtained from red-producing cochineal scale insects (21).

Figure 6. The alizarin–aluminum–calcium complex based on the work of Wunderlich and Bergerhoff (20). 203 Rasmussen; Chemical Technology in Antiquity ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


With all of the differing proposed molecular structures given in the literature for this complex, more research in this area would be most welcome. Whatever the correct structure is for these mordant dyes in wool, it is remarkable that the ancient dyer was producing metal complexes and utilizing the coordination chemistry associated with chelation several millennia before Alfred Werner (1866–1919) expounded on coordination compounds and Gilbert Newton Lewis (1875–1946) explicated his universal concepts of acid-base properties. A beautiful example of the aluminum-alizarin complex that the ancient dyer created is from the 2nd century CE Roman Period (Figure 7).

Figure 7. Fragments from a multicolored 2nd century CE scroll wrapper from the period of the Bar Kokhba revolt found in the Cave of Letters, Judean Desert; the red background was dyed with madder containing mostly alizarin and complexed with an alum mordant. 204 Rasmussen; Chemical Technology in Antiquity ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Redox Purple Chemistry of the Ancients


Fermentation and Vatting As impressive as the ability of the ancient dyer to create coordinated complexes is, it is the dyeing with indigoid compounds from purple-producing sea snails that really showed off the chemical prowess of this artisan. These purple dyeings are famously renowned for the regalia of kings, priests, and the military, and referred to by various names, such as Royal Purple or Tyrian Purple. Direct archaeological evidence of dyeing with molluskan purple is based on various purple-stained and analyzed potsherds from dyeing vats, and dates back to at least three and a half millennia when the Levantine Phoenicians developed this chemical technology (13). Performing a dyeing from the purple pigment produced from certain mollusks was similar to – but much more biologically challenging – than dyeing with the indigo pigment (also referred to as indigotin). The latter colorant was produced from various precursors in the leaves of certain plants, such as Indigofera tinctoria (the indigo plant, so-named because it is native to India and environs), and also from woad (Isatis tinctoria), a plant that was available in the ancient Near East and Europe (see Figure 8).

Figure 8. Left: the woad plant, Isatis tinctoria (Wikimedia commons). Right: the author holding a dark blue-violet indigo pigment produced from the isatans in the leaves via fermentative reduction and air-oxidation.

Photochemical Oxidation Production of the Purple Pigment The pigment produced from most purple-producing Mollusca is of a reddish-purple coloration, an example of which is given in Figure 9. However, while a type of the Hexaplex trunculus sea snail species (also known as Murex trunculus), the most important purple-producing mollusk of the Eastern Mediterranean, can produce that coloration, another chromatic subgroup of H. trunculus mollusks produces bluish-purple or violet pigments. In any case, though the purple pigments from different sea snail species may contain different colorants, all purple-producing mollusks, whether from the waters off Japan, the 205 Rasmussen; Chemical Technology in Antiquity ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Atlantic Ocean, or the Mediterranean Sea, possess the same common chromatic marker, the red-purple 6,6′-dibromoindigo (abbreviated as DBI) colorant (13). The formation of the colorants in the purple pigment from the colorless precursors contained in the hypobranchial gland of the living animal consists of complex steps (22) that can be schematically outlined for the case of DBI, as shown in Figure 10. Each precursor is hydrolyzed by purpurase, a sulfatase enzyme, which is present in the gland but is not in contact with these precursors as long as the snail is alive. When the snail expires or when the gland is cut or pierced, then the enzyme comes in contact with these precursors, which after hydrolysis followed by air-oxidation and photolytic processes, produce the indigoids, such as the DBI shown in the figure.

Figure 9. Left: an expiring Hexaplex trunculus sea snail expelling the colorless precursors from its hypobranchial gland, which under the influence of air and light, undergo photochemical oxidation to the red-purple pigment. Right: the molecular structures of the three indigoid components in the H. trunculus pigment (top to bottom): indigo (IND), 6-bromoindigo (also known as monobromoindigo, MBI), and 6,6′-dibromoindigo (DBI), the latter being the most abundant component of any red-purple colored pigment. Not all snails were created equal. The major component in all red-purple molluskan pigment from all species is DBI, and it can even be more than 90% of the dye content in such pigments. However, H. trunculus sea snails produce other indigoids in varying amounts besides the red-purple DBI colorant, and these are the violet MBI and the dark blue IND colorants. This is summarized in Figure 11. The presence of a significant quantity of the violet MBI colorant is unique to H. trunculus snails and finding appreciable quantities of it in a purple pigment – whether archaeological or modern – indicates that its malacological dyestuff source is from H. trunculus. In fact, MBI may even be the colorant in greatest abundance in such pigments. Further, the pigments from H. trunculus snails can be categorized in two ways, depending on the relative quantities of DBI and IND in their pigment. Thus, one type produces a red-purple pigment because it is richer in DBI, while the other type is richer in IND and subsequently the pigment’s color is blue-purple or violet. 206 Rasmussen; Chemical Technology in Antiquity ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 10. An outline of the spontaneous production of the DBI pigment via enzymatic hydrolysis of a precursor in Hexaplex trunculus sea snails followed by oxidative coupling and photochemical reactions.

Figure 11. The three Muricidae family of sea snails from which purple pigments were produced in antiquity (from left to right), Hexaplex trunculus, Bolinus brandaris, and Stramonita haemastoma, and the main colorants that they produce. 207 Rasmussen; Chemical Technology in Antiquity ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Fermentative Bacterial Reduction All the indigoid colorants in the purple pigment are relatively water-insoluble colorants – pigments – due to the strong intra- and inter-molecular hydrogen bonds that they form, and the solubility decreases as more Br atoms are attached to the indigo skeleton. Thus, for any textile dyeing to be performed – whether natural or synthetic – the colorant must be dissolved. In the case of an indigoid, this dissolution is accomplished via reduction – or “vatting” – of the pigment to yield the reduced and less-colored or “whiter” dye known as “leuco”. The natural reducing agent to effect the solubilization of the solid pigment via fermentation is the thermophilic bacteria present in – and feeding off – the meaty glands of the rotting flesh (23). The first step in the reduction process produces the non-ionized leuco acid, which is only sparingly soluble (Figure 12). The affinity of the reduced dye to the textile fibers will be significant when the colorant is in a soluble and ionic state, and thus the reduction is performed in an alkaline environment to produce the soluble monoanion (24). If the pH is further increased then the more soluble dianion can also exist in appreciable amounts, but the pH should not exceed 9 when dyeing a proteinic material, such as wool, so as not to cause degradation of the textile.

Figure 12. Fermentative bacterial reduction of the indigoids to their leuco species as a function of the alkalinity of the solution; the actual dyeing is effected by introducing the textile in the dye vat and after a few hours removing it into the air whereby it undergoes air-oxidation to the original pigment’s components. The fractions of the reduced brominated indigoids existing in equilibrium as a function of pH has not yet been researched, which would be of great interest, however the relevant quantities of the different species of indigo itself have been 208 Rasmussen; Chemical Technology in Antiquity ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


studied. The two equilibrium dissociation constants for reduced indigo have been originally estimated from other data and reported to be 8.0 and 12.7 for pK1 and pK2, respectively (24). From an interpolation and extrapolation of the relevant curves in recently published graphs (25), it is found that the value for pK1 is different and estimated at 9.45, while pK2 is the same 12.7 value. Based on the more recent values for the dissociation constants of the reduced indigo species, the fraction of each species as a function of the pH can be evaluated from published equilibrium equations (25), and the classical mirror-image α-shaped curves are obtained (Figure 13). The diagram shows that at a working pH of 9, about 20% of the reduced indigo is in the monoanion state, whereas about 80% is the nonionized leuco acid species. However, from the recently published graphs (25), there is a steeper rise in the monoanion composition so that its fraction is at 20% already at pH 8. In any case, the monoanion species necessary to bond to the wool fibers is present under alkaline conditions. Similar results should be obtained for the reduced brominated indigo species.

Figure 13. Mirror-image α-diagram showing the reduced indigo fractions based on the pK1 = 9.45 and pK2 = 12.7. H2I refers to the reduced (leuco) indigo acid, HI– and I2– are the respective mono- and di-anions of reduced indigo. The diagram shows that the dianion is indeed irrelevant at the moderate alkaline pH values that are needed so as not to destroy the proteinic wool structure. It is the monoanion that is substantive and attaches itself to the wool fibers via various intermolecular bonds. Hence, in effect, the relevant equilibrium is mainly between the reduced nonionic indigo (represented by “H2I”) and its monoanion, HI–:

Though, the mono-anion’s presence in the reduced dye vat is much less than the nonionic species, nevertheless as more of the monoanion is removed from the solution and bonds to the fibers, then the equilibrium of the conjugate acid209 Rasmussen; Chemical Technology in Antiquity ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

base pair shifts to produce more of the monoanion from the nonionic acid. This right-shift in the equilibrium was practiced by the advanced ancient chemical dyer thousands of years before the famous French chemist, Henry Louis Le Châtelier (1850–1936), invoked his principle on the “lois des équilibres chimiques”.


Redox Dyeing After the purple pigment was reduced and dissolved, the actual dyeing could be performed. The overall dyeing process then follows a reduction-oxidation cycle, which is schematically shown in Figure 14. The anaerobic bacterial reduction requires that the vat – the clay vessel containing the dye bath – be covered to minimize the entrance of air into the solution. A few days of fermentation are normally required for the natural reduction of the pigment to occur, and this can be clearly observed as the purple-color of the aqueous mixture transforms to the green-colored solution (23). At this point, a cleaned woolen fleece is inserted into the solution and kept at a temperature of about 50 – 60 °C for a few hours. Afterwards, as the green-colored wool is removed from the solution and into the air, the reduced species in and on the fibers immediately begin to undergo oxidation by the atmospheric oxygen to the original purple pigment, though not exactly with the same composition as in the raw pigment.

Figure 14. Overall dyeing process including the pre-dyeing reduction stage, wool insertion into dye bath, followed by air-oxidation of the reduced indigoids. The essence of this dyeing strategy was in getting the pigment, which originally was not connected to the textile, to impregnate the fibers, which could have only occurred in the dye’s dissolved state. Then, when air-oxidized to the pigment again, this pigment is now “imprisoned” inside the interior of the fibers, 210 Rasmussen; Chemical Technology in Antiquity ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


and is also attached to the external fibers, by means of various intermolecular forces. In short, the colorant is a pigment before and after the dyeing, but in the middle stage it is in a water-soluble reduced state as a dye. A most beautiful example of such a Royal Purple dyeing is from a miniscule weave fragment that is probably from the royal robe, cloak, or mantle of the 1st century BCE King Herod the Great (26), and shown in Figure 15.

Figure 15. A close-up of the miniscule fragment of only a few millimeters showing the purple-dyed yarns from the1st century BCE Herodian fabric found atop Masada in the Judean Desert; the few beige-colored yarns are undyed woolen fibers that have yellowed and been sullied over the archaeological time frame.

Analytical Methods of Dye Analysis An investigation of ancient chemical technologies requires a three-pronged approach that incorporates historical accounts regarding dyeing technologies, physical archaeological findings, and scientific analyses on these historic artifacts as well as on modern dyestuffs that would have also been in existence in antiquity. The starting point for this study is, then, a dilemma, since the relevant question is “where best to start?”. However, because the triad topics are interconnected any 211 Rasmussen; Chemical Technology in Antiquity ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


of the subjects can be approached first as one investigation leads to the others and back again to the first. A necessary stage in the investigation is to identify the dyestuff sources discussed in the historic literature and be hopeful that they also exist in modern times so that these can be analyzed via advanced analytical instrumentation. Scientific analyses of these sources will produce a chemical library of the various colorants in each dyestuff. Towards this step, it is crucial to use the correct analytical instrumentation for such a purpose. Unfortunately, in some cases, various instrumental techniques have been used for dye analyses without an understanding of these limitations. Not every instrumental technique – advanced as it may be – is automatically useful for analyses of these colorants (27). The main limitation of all spectrometric techniques is that they produce an “overlap of data”, i.e., these techniques measure the bulk sum of all species, which can result in an overlap of information from different components. The latter property is crucial to unambiguously identify the exact dyestuff species because every natural dye source consists of several components. Analytical instrumental chemistry is inundated with a myriad of acronyms representing many advanced methods of analyses. Accordingly, the analytical chemist is faced with such abbreviations as NDT, NDI, FORS, FAB, DESI, DART, HRMS, TOF, SERS, TERS, DRIFTS, DAD, and the list goes on. With all these methods, it can be confusing as to which technique to use for dye analyses, and a variety of techniques have indeed been used. In this regard, it is important to differentiate between inorganic pigments, which are typically characterized by a metallic entity that can be relatively easily analyzed via various elementary spectrometric techniques (such as, XRF, SEM-EDX, AA, ICP, etc.), and organic pigments and dyes. An additional dilemma for museum officials – the “Curator’s Quandary” – is whether to use a destructive technique or a non-destructive one. This Shakespearean predicament – “to destroy or not to destroy?” – is the question that curators and conservators must ask themselves. It is well acknowledged for many years now that the optimal method for dye analyses is HPLC (high-performance liquid chromatography) – or its more modern “ultra” version, UPLC (also referred to as UHPLC). This technique provides the most detailed information regarding the various dye components constituting a dyestuff or pigment. Though this is a destructive technique, it is micro-destructive – actually nano-destructive – in that it has been shown that it can produce detailed results on even a single fiber of a few millimeters in length representing nanogram levels of dye (28). There is no other technique that can compete with LC for the full analysis of the dyestuffs and pigments. Thus, an Ecclesiastical answer to the Shakespearean question can be that “there is a time to build, and a time to destroy”, and with the nano-destructive LC method, producing the most detailed results far outweighs the negligible invasiveness of this technique (29). An example of the detailed information produced by HPLC is the chromatogram obtained for the various colorants that may be present in a molluskan purple pigment (Figure 16). The chromatogram curve shows the separation of each component at the time that the mobile phase elutes it out of the stationary phase – known as the retention time (R.T. or tR) – and the absorbance 212 Rasmussen; Chemical Technology in Antiquity ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


at this time. In essence, a chromatic “fingerprint” of the dyestuff is produced so that not only a qualitative analysis is obtained of what colorants are present in the dyestuff, but also a quantitative analysis indicates how much of each colorant is present. Metaphorically, the human analogy is that the peaks in the chromatogram represent fingers and their position in the chromatogram show which fingers they are; the heights (technically the areas under each peak) indicate the lengths of these fingers. For example, the red-purple and blue-purple (or violet) pigments produced from H. trunculus snails can contain about 10 colorants, with yellow, orange, blue, violet and red-purple colors as shown in the figure (30).

Figure 16. HPLC-produced chromatogram of the various indigoids (IND, MBI, and DBI) and related components, with their representative colors, which may be present in a molluskan purple pigment. The other components are: IS = isatin, 4BIS = 4-bromoisatin (not present in a molluskan pigment but included for standardization), 6BIS = 6-bromoisatin, INR = indirubin, 6MBIR = 6-(mono)bromoindirubin, 6′MBIR = 6′-(mono)bromoindirubin, and DBIR = 6,6′-dibromoindirubin; the right absorbance scale is for the first three peaks representing the isatinoids. Together with the spectrometric detection of each peak, which produces a UV/Visible spectrum with the PDA (photo-diode array) detector (also abbreviated as DAD), these two properties – chromatographic retention time and spectrometric – can be used to positively identify the colorants present when compared to a standardized library of analytical results performed on the possible natural colorants. An additional detector that is helpful in the dye identifications would be MS (mass spectrometric). 213 Rasmussen; Chemical Technology in Antiquity ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


A normalized quantification has been made (Figure 17) on the three abovementioned Murex sea snails that inhabit the Eastern Mediterranean – including the two chromatic species of the H. trunculus snails – and compared with an archaeological pigment found on a King Darius marble jar from 2,500 years ago (31). To date, all the archaeological purple pigments that have been properly analyzed via HPLC have shown the significant presence of MBI. This clearly indicates that H. trunculus snails were used in all the dyeings and paintings, alone or sometimes together with the other Muricidae snails, in all archaeological purple pigments found to date.

Figure 17. A multi-component normalized quantification of the integrated peak areas (measured at the standard 288 nm wavelength), which can be used as semiquantitative measures of the relative amounts of the dyes in the purple pigments from various sea snail sources; (bottom, left to right) red-purple paint residue on a 2,500 year old King Darius marble jar, red-purple and blue-purple pigments from modern H. trunculus, and red-purple pigments from modern B. brandaris and S. haemastoma. 214 Rasmussen; Chemical Technology in Antiquity ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Conclusions


The production of colorfully dyed textiles from more than four millennia ago required an advanced degree of practical chemical knowledge that was based on inorganic, organic, and physical chemistry, and biochemistry. As such, the long-lasting stable dyeings that we are witness to nowadays are the results of strong intermolecular bonds between the colorants and the textile fibers that the ancient dyer created. Today, with all of the accessible modern tools, we can only marvel at the high degree of empirical chemistry that the ancients practiced. In their hoary past, with only primitive tools and primeval conditions, they exhibited a modern mastery of chemical principles. These ancient dyers were truly Nobles of Chemistry.

Acknowledgments The author would like to express his sincere appreciation to the Sidney and Mildred Edelstein Foundation for support of this work and to Seth Rasmussen for the invitation to deliver this paper at the American Chemical Society symposium on ancient chemical technologies.

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